When most people think of extreme athletes — Ironman triathletes, marathoners, and even those who compete in 100-mile ultra marathons — the picture of a very fit, very healthy person comes to mind.

But people who consistently run 15- to 20-miles as often as three to five days a week might be doing more harm to their hearts than good, said Dr. Barry Franklin, PhD., director of preventive cardiology and cardiac rehabilitation at Beaumont Health in Royal Oak, Mich.

“A lot of people think that more exercise is invariably better. … That is not necessarily so,” he said.

“You can overdose exercise. Our job is to find what’s the sweet spot, what’s enough intensity to get you benefit that keeps you safe?”

Franklin seconds the recommendation of the U.S. Centers for Disease Control and Prevention, which suggests 150 minutes of moderate exercise each week or about 60 to 70 minutes of vigorous exercise weekly for adults.

“The person who benefits the most from an exercise program is the one who currently falls in the least fit, least active cohort, or what we call the bottom 20%,” he said.

Franklin says there are five surprising things about heart health and exercise:

1. High-intensity interval training is risky for anyone over age 40

“I don’t recommend, do not recommend, that middle age people or older go out and do high-intensity interval training, where you’re sprinting” for a short time, followed by a recovery period and then sprinting again, Franklin said.

“If you’re 20, 25 years of age and you’ve got no risk factors, that’s fine,” he said. “If you’re 40, 50, 60, 70, I don’t think there is evidence that shows that the benefits outweigh the potential risk, especially if they’re doing it in a non-medical setting like the YMCA.

“Everybody who develops heart disease doesn’t always have risk factors. The vast majority do, but just because you don’t have risk factors doesn’t guarantee that you can’t have underlying heart disease.

“It’s not the duration generally or the frequency of exercise that gets people in trouble. It’s the intensity. I would err on the conservative side and say I’d rather see you exercising at more moderate intensities and … exercising more frequently or days per week or for longer periods.”

2. Deer hunting can put you at higher risk for heart attack

It’s a combination of factors — cold weather, the thrill of the hunt and sedentary lifestyle — that puts many deer hunters at risk for heart attacks in the field.

“We’ve done some studies here at Beaumont showing very high heart rates, blood pressures,” Franklin said. “We have one case report showing a guy’s heart rate standing next to a tree went from 70 up to 170 (beats per minute) simply by seeing a deer in front of him.

“So what’s that about? He became so excited that by the time he pulled the trigger, his heart rate took off and went up to 170. Oftentimes, deer hunting involves people who are habitually sedentary, generally guys who go out once a year or twice a year to deer hunt. Oftentimes they’re eating, drinking, smoking. It’s cold weather. They are wearing heavy clothes, carrying a gun, climbing up hills, which they don’t normally do, and, lastly, the highest heart rates we found were when they killed a deer and were actually trying to drag that deer out of the woods.”

3. Your risk of heart attack goes up during exercise, but being sedentary is worse

“On average, the relative risk of a heart attack or sudden death for the general population increases four- to five-fold during vigorous exercise,” Franklin said. “So if you go out and you run around the track, your risk of heart attack or sudden death is four- to five-fold over sitting there, reading your morning newspaper.

“But there’s big variability in that. … The relative risk increases more than a hundred-fold in sedentary people who go out and do unaccustomed high-intensity exercise. So if you’re sitting at a computer or on your couch all year and do nothing and then all of a sudden, you get a winter snowstorm and you go out and start shoveling heavy wet snow, that’s the person who’s at the greatest risk.

“The risk increases only about two-fold when you go out do vigorous exercise if you’re regularly doing high levels of habitual physical activity.”

But that doesn’t mean it’s OK to be a couch potato, either. Exercise in moderation, he said, has oodles of benefits, including mood-boosting, anti-aging effects, and has been shown to reduce cholesterol and blood pressure.

“Research has clearly shown that as fitness improves mortality, death rates, decrease up to a point and then up to a point is an important point,” Franklin said, “beyond, which there appears to be little or no additional survival benefit.”

4. That final mile is a doozy

The New England Journal of Medicine published a 2012 study of 10.9 million U.S. marathon and half-marathon runners over a 10-year period.

Between 2000 and 2010, very few runners died during marathons and half marathons — just 1 per 259,000 participants. In all, there were 59 cardiac arrests. Of them, 42 were fatal.

“What’s interesting is that the final mile of a marathon accounts for about 50 percent of the deaths,” Franklin said. “It occurs in people who’ve been running two, three, four or five hours. Their electrolytes are low, their sodium and potassium levels may be imbalanced. Their temperature is high, their heart rate has really taken off, and then they decide they’re going to sprint the last hundred yards or 200 yards to get the best time.

“So when marathon directors say to me, where should we have our EMS trucks, I say they should be primarily over the last mile of the run.”

Of the runners who died, the No. 1 autopsy finding was hypertrophic cardiomyopathy, which is an enlarged heart. No. 2 was clogged coronary arteries or atherosclerotic cardiovascular disease.

“We can oftentimes determine whether somebody has an enlarged heart or underlying heart disease and can tell them that they probably shouldn’t be out doing vigorous exercise at this time,” Franklin said, noting that screenings at the Beaumont Cardiovascular Performance Clinic cost about $370.

5. Slow walking is a harbinger of death in middle and old age

You might want to pick up the pace the next time you go for a walk.

“We now realize that even somebody’s walking speed is a good index of their overall health,” Franklin said. “Several studies have now shown that if middle age and older adults in their day-to-day activities walk at less than 2 miles an hour, we say that’s a harbinger of the approaching grim reaper, the personification of death. Those people are headed toward generally a poor prognosis.

“Whereas numerous studies now show that middle age and older adults who walk briskly — 3 miles an hour or faster — generally have very good health profiles. So I tell our patients, invariably, you can disguise your exercise by picking up your pace, simply walking on day-to-day basis at 3 miles an hour or faster.”